1. Field of the Invention
The present invention relates to a method and an apparatus for determining exertion levels in fitness or athletic training and for determining the stress caused by training by means of heartbeat rate measurement.
2. Description of the Prior Art
Heartbeat rate measurement is used in fitness and athletic training for determining working capacity or a stress level. This is based on the observation that an almost linear dependency exists between heartbeat rate and increasing working capacity within the main exertion area. The change in the energy production processes of vital functions, such as respiration, heartbeat rate and muscular action, i.e. metabolism, is generally used in determining exertion levels, for instance by means of lactic acid, which is a waste material of the energy production processes.
When a person starts the training, the lactic acid content of muscles and blood falls at first slightly below the starting level. This is due to stimulated circulation, and to lactic acid decomposition and removal processes improving with muscular action. Thereafter, when the exertion level is steadily increased, the lactic acid content starts to increase linearly in relation to the stress. The working capacity at which the lactic acid content reaches its starting value is referred to as the aerobic threshold, at which corresponding vital function levels, such as heartbeat rate frequency and oxygen consumption, can be determined.
At working capacities lower than the aerobic threshold, energy is produced aerobically, i.e. by burning mainly fats and a small amount of carbohydrates by means of oxygen. At working capacities higher than the threshold, energy is produced to an ever-increasing degree anaerobically, i.e. in a manner involving an oxygen debt, whereby more and more lactic acid is correspondingly produced in the tissues. At the same time, the proportional amount of the use of fats in energy production decreases, and carbohydrates become the main source of energy.
When the performance level is further increased, a situation finally arrives where the system is no longer capable of processing the amount of lactic acid produced in energy production. The disturbance of the balance is detected as an abruptly increasing content of lactic acid in the blood and as a distinct increase in lung ventilation, taking place at the same time. This exertion level, which is significant as regards training, is referred to as the anaerobic threshold.
The exact determination of the aerobic and anaerobic threshold is carried out today in special test laboratories by means of a maximum stress test based on breathing gas analysis. The test is started with a small stress. The stress is increased without an intermission by small steps at intervals of 2-3 minutes all the way to the maximum exertion. From the last 30 seconds of each stress level, the ventilation, used oxygen, and produced carbondioxide are registered, and a blood sample is taken to determine the lactic acid level. The threshold values are determined by means of the lactic acid and the breathing parameters. However, the results are clearly affected by the manner of applying the stress, the speed of increasing the stress, and the stress levels used, whereby threshold values of different magnitudes are obtained in different physical stress situations, for instance with the use of a running mat and an ergometer. On the other hand, the repeatability of the aerobic and anaerobic thresholds is questionable; for instance in increasing the stress in a sliding manner and not by steps, the threshold phenomena can not be detected. Reference is made for instance to U.S. Pat. No. 5,297,558, according to which a person must be subjected, in determining the exertion level of the person, to exertion increasing by steps during the training period in order that the points of discontinuity in the heartbeat rate variation graph, indicating the limits of different exertion levels, would appear sufficiently clearly.
Today, the determinations of the aerobic and anaerobic thresholds are primarily used in training endurance athletes to determine suitable training capacities and to monitor the effects of the training. Similarly, the thresholds can be used for determining optimum training capacities e.g. for a dieting person. When heartbeat rate values corresponding to the thresholds are known, a desired training capacity can be accurately maintained by means of continuous heartbeat rate monitoring. The presently available methods for determining the threshold values are based on difficult lactic acid or breathing gas measurements, in which the taking of blood samples, expensive laboratory equipment and staff requirements play a significant part. Furthermore, the determinations are based on a phenomenon the repeatability of which has not been possible to verify.
The aerobic and anaerobic thresholds are thus mainly used in monitoring the performance level of athletes. As regards a person engaged in fitness training, the situation is entirely different. Few people have the possibility to have expensive laboratory measurements to determine the threshold values. On the other hand, the needs of a person engaged in fitness training differ from those of athletes. The correct exertion level of a person engaged in fitness training is represented by exercise which is sufficient for maintaining health and improving fitness but which does not exceed the limit of safe exertion. The need for anaerobic training is small, and aerobic training is emphasized. For the needs of persons engaged in fitness training, a heartbeat rate range can be divided into four target heartbeat rate ranges. The lowest capacity range, within which the heartbeat rate is not more than 55% of the maximum heartbeat rate, does not yet provide a sufficient training response. In moving within a capacity range of 55-65% of the maximum heartbeat rate, an efficient use of fats starts. The actual target heartbeat rate range in aerobic training, 65-85% of the maximum heartbeat rate, provides the best final result in target-oriented fitness training. With a heartbeat rate higher than this, energy production rises partly to a distinctly anaerobic area. In the presently widely used method for determining the target heartbeat rate ranges, the correct exertion levels are determined as based on the measured or estimated maximum heartbeat rate of a person engaged in fitness training. The maximum heartbeat rate can be accurately measured during extreme exertion, but this may cause a health risk especially as regards beginners in fitness training. Generally, the maximum heartbeat rate and training heartbeat rates are estimated on the basis of the age of the person engaged in fitness training from a calculation formula, or by means of the resting heartbeat rate and the maximum heartbeat rate by the so-called Karvonen method. Since maximum and resting heartbeat rates are individual, the error margin of the estimation methods is great. On the other hand, the threshold values are not fixed but they vary for instance when the level of fitness changes. For instance, the anaerobic threshold corresponds to about 60-75% of the maximum heartbeat rate with persons who have not trained and who are in poor condition. For an athlete who has trained for a long time and who is in top condition, the threshold may be even more than 80% of the maximum heartbeat rate. Due to this, the definition of target heartbeat rate ranges fails for a majority of persons engaged in fitness training, and the advantages of the method remain scarce.
When contracting, a heart produces a series of electric pulses, which can be measured everywhere in a body. The measurement and analysis of such a signal is referred to as electrocardiography (ECG). The actual signal is referred to as an ECG signal. In an ECG signal, it is possible to distinguish phases resulting from different operational stages of the heart. These portions are the so-called P, Q, R, S, T and U-waves (cf. FIG. 1), which will be described in more detail below.
Due to the variation in the sympathetic-parasympathetic balance of the autonomic nervous system, variations around the average heartbeat rate level occur constantly in heartbeat rate. The variation in heartbeat rate is caused by the function of the cardiovascular control system. The main reasons for the variation are respiratory arrhythmia, variation caused by blood pressure control, and variation caused by the heat balance control of the system. Among these, the most significant and causing the greatest variation is respiratory arrhythmia. The transmitting nervous systems of heartbeat rate variation can be distinguished by means of heartbeat rate variation frequency analysis. At the present time, the sympathetic nervous system is considered to be slow; it is hardly capable of transmitting frequencies higher than 0.15 Hz. Instead, the operation of the parasympathetic nervous system is fast, wherefore frequencies higher than the above-mentioned threshold frequency are transmitted through the parasympathetic nervous system.
Heartbeat rate variation can be measured for instance by means of standard deviation. Other generally used variation measuring units are spectrum calculation power values, the maximum value of the variation, and the height of the deviation diagram. The standard deviation does not distinguish the frequency components of intervals between R-wave peaks, i.e. R--R intervals, but it is affected by frequencies transmitted from both autonomic nervous systems. In measuring the standard deviation of short-term R--R-intervals during exertion, it can be stated, justifiably and in a partly simplified manner, that the standard deviation measures almost exclusively the proportion of parasympathetic control in the heartbeat rate variation. This is mainly due to the fact that the greatest source of variation (respiratory frequency) during exertion rises unavoidably higher than those frequencies which the sympathetic nervous system is capable of transmitting.
In increasing the exertion level from the resting level, the parasympathetic tonus decreases at first by degrees. When the heartbeat rate level has risen to a level of about 100 pulsations/min, i.e. to about 56% of the maximum heartbeat rate, the sympathetic activity starts to rise, and will have a significant effect on the heartbeat rate frequency at a level of about 63% of the maximum heartbeat rate. With low exertion, an increase in the heartbeat rate is almost completely due to decreased parasympathetic activity. The heartbeat rate variation thus decreases in a direct proportion to the disappearance of the parasympathetic control. It is only on a higher exertion level that the sympathetic nervous system participates in controlling the heartbeat rate level with the parasympathetic one.
Respiratory arrhythmia has been found to be the most important cause of heartbeat rate variation. The strength of respiratory arrhythmia depends both on the deepness of respiration and respiratory frequency. The maximum amplitude is reached at a respiratory frequency of 5-7 times per minute. Furthermore, respiratory arrhythmia attenuates strongly as the respiratory frequency increases. The deepness of the respiration affects the strength of arrhythmia up to a level corresponding to 50-60% of the respiratory capacity. At a volume higher than this, the respiratory arrhythmia no longer increases. During fitness training, the respiratory volume always exceeds this limit point, wherefore the effect thereof can be ignored in this context. The heartbeat rate variation during a performance can thus be regarded as representing the respiratory frequency of the person engaged in fitness training to a large extent. Furthermore, empirical knowledge concerning fitness training is that if a person engaged in fitness training is able to carry on a conversation during his performance, the exertion level is suitable and the fitness training is aerobic. The energy requirement of the system can thus be satisfied without anaerobic energy production, and lactic acid causing subsequent soreness of muscles does not accumulate in the system. On the other hand, if the person engaged in fitness training is able to speak, his entire respiratory capacity is not used. Relaxed respiration causes relatively wide heartbeat rate variation. If the exertion level of the person engaged in fitness training approaches the anaerobic area, he must unavoidably start using his entire respiratory capacity. The respiration is smooth and quick-paced, heartbeat rate variation thus remaining low.